Artificial ground freezing of underground mines in cold regions using thermosyphons with air insulation

Current practice of underground artificial ground freezing(AGF)typically involves huge refrigeration systems of large economic and environmental costs.In this study,a novel AGF technique is proposed deploying available cold wind in cold regions.This is achieved by a static heat transfer device called thermosyphon equipped with an air insulation layer.A refrigeration unit can be optionally integrated to meet additional cooling requirements.The introduction of air insulation isolates the thermosyphon from ground zones where freezing is not needed,resulting in:(1)steering the cooling resources(cold wind or refrigeration)towards zones of interest;and(2)minimizing refrigeration load.This design is demonstrated using well-validated mathematical models from our previous work based on two-phase enthalpy method of the ground coupled with a thermal resistance network for the thermosyphon.Two Canadian mines are considered:the Cigar Lake Mine and the Giant Mine.The results show that our proposed design can speed the freezing time by 30%at the Giant Mine and by two months at the Cigar Lake Mine.Further,a cooling load of 2.4 GWh can be saved at the Cigar Lake Mine.Overall,this study provides mining practitioners with sustainable solutions of underground AGF.

Performance analyses of two-phase closed thermosyphons for road embankments in the high-latitude permafrost regions

Two-phase closed thermosyphons(TPCTs)are widely used in infrastructure constructions in permafrost regions.Due to different climatic conditions,the effectiveness of TPCT will also be different,especially in the extremely cold region of the Da Xing'anling Mountains.In this study,a series of three-dimensional finite element TPCT embankment models were established based on the ZhanglingMohe highway TPCT test section in Da Xing'anling Mountains,and the thermal characteristics and the cooling effect of the TPCTs were analyzed.The results indicated that the TPCTs installed in the northeastern high-latitude regions is effective in cooling and stabilizing the embankment.The working cycle of the TPCTs is nearly 7 months,and the cooling range of the TPCTs can reach 3 m in this region.However,due to the extremely low temperature,the TPCT generates a large radial gradient in the permafrost layer.Meanwhile,by changing the climate conditions,the same type of TPCT embankment located in the Da Xing'anling Mountains,the Xiao Xing'anling Mountains,and the Qinghai-Tibet Plateau permafrost regions were simulated.Based on the comparison of the climate differences between the Qinghai-Tibet Plateau and Northeast China,the differences in the effectiveness of TPCTs were studied.Finally,the limitations of using existing TPCTs in high-latitude permafrost regions of China were discussed and the potential improvements of the TPCT in cold regions were presented.

Theoretical and Experimental Investigations of Solar Heating Systems at Specified Output Conditions of Hot Water

The theoretical analysis discussed in this work is a suitable mathematical tool by which the performance of the proposed collector can be predicted. The obtained experimental results coincide with the obtained theoretical data obtained from the devised computer program. Controlled output temperature can be obtained from the proposed system. The performance of the tested collector under the proposed intermittent flow conditions overcomes that of the conventional thermosyphone flow collector.

Causes and detection of coalfield fires, control techniques, and heat energy recovery: A review

Coalfield fires are considered a global crisis that contributes significantly to environmental destruction and loss of coal resources and poses a serious threat to human safety and health. In this paper, research related to the initiation, development, and evolution of coalfield fires is reviewed. The existing detection and control techniques of coalfield fires are also reviewed. Traditional firefighting is associated with waste of resources, potential risks of recrudescence, potential safety hazards, extensive and expensive engineering works, and power shortages. Recently,coalfield fires have been recognized as having significant potential for energy conservation and heat energy recovery. Thermoelectric power generation is regarded as a suitable technology for the utilization of heat from coalfield fires. The extraction of heat from coalfield fires can also control coalfield fires and prevent reignition leading to combustion. Technologies for absorbing heat from burning coal and overlying rocks are also analyzed. In addition, the control mode of "three-region linkage" is proposed to improve firefighting efficiency. Integrating heat energy recovery with firefighting is an innovative method to control coalfield fires.

Laboratory test on the combined cooling effect of L-shaped thermosyphons and thermal insulation on high-grade roadway construction in permafrost regions

With the completion of the Qinghai-Tibetan Railway,economic development of related areas has been greatly accelerated.This,in return,calls for building or upgrading more roadways,especially high-grade roadways.In cold regions,the thawing of permafrost can induce settlement damage of and even failure to railway (or roadway) embankments.Thermosyphons (self-powered refrigera-tion devices that are used to help keep the permafrost cool) have proved effective in mitigating thaw settlement by maintaining the thermal stability of the embankments.However,for high-grade roadway embankments of great width,stabilizing or cooling ef-fects of traditional geotechnological measures may be limited.To enhance the cooling effect of thermosyphons,an L-shaped thermosyphon was designed.A laboratory test was carried out to study the combined cooling effect of the L-shaped thermosyphon and thermal insulation applying to roadbed construction.The angle between the evaporator and condenser sections of the L-shaped thermosyphon is 134 degrees,and the L-shaped thermosyphon was inserted into the soil at an angle of 5 degrees with the road surface.The tested results show that the L-shaped thermosyphon is effective in removing heat from a roadway in winter.When the ambient air temperature is lower than the soil temperature,the thermosyphon is active and extracts the heat in the soil around it.When the ambient air temperature is higher than the soil temperature,the thermosyphon is inactive,and no heat is in-jected into the soil through the L-shaped thermosyphon.Compared to embankments with straight thermosyphons,the inner parts of the embankments with L-shaped thermosyphons were significantly cooled.It is hoped that the present study would be useful to the application of L-shaped thermosyphons in the construction of high-grade roadways in cold regions.

CONDENSING HEAT TRANSFER IN AN ADVANCED TWO-PHASE CLOSED THERMOSYPHON

1 INTRODUCTIONClosed thermosyphon has been developed to enhance heat transfer and recover wasteheat in various process industries [1,2].Stimulated by this success,a new type oftwo-phase closed thermosyphon was designed by inserting respectively two inner tubesinto the thermosyphon,one in the boiling section and the other in the condensing sec-tion.The two-phase flow boiling heat transfer coefficient was calculated successfully onthe basis of Chen’s dual-mechanism [3].A boiling heat transfer model for thetwo-phase closed thermosyphon with an inner tube in the boiling section was pro-

Experimental study on transient behavior of semi-open two-phase thermosyphon

An experimental system was set up to measure the temperature, pressure, heat transfer rate and mass flow rate in a semi-open two-phase thermosyphon. The behaviors of a semi-open two-phase thermosyphon during startup, shutdown and lack of water were studied to get complete understanding of its thermal characteristics. The variation of wall temperature, heat-exchange condition and pressure fluctuations of semi-open two-phase thermosyphons showed that the startup of SOTPT needs about 60-70 min; the startup speed of SOTPT is determined by the startup speed of the condensation section; the average pressure in the heat pipe is equal to the environmental pressure usually; the shutdown of SOTPT needs about 30-50min; a semi-open two-phase thermosyphon has good response to lack of water accident.

Centrifuge model test on performance of thermosyphon cooled sandbags stabilizing warm oil pipeline buried in permafrost

The thaw settlement of pipeline foundation soils in response to the operation of the first China-Russia Crude Oil Pipeline along the eastern flank of the northern Da Xing'anling Mountains in Northeast China was simulated in a physical model test(with a similitude ratio of 1/73) in a geotechnical centrifuge. Two pipes of a supported and an unsupported section were evaluated over a testing period for simulating 20 years of actual pipeline operation with seasonal cyclically changing oil and ambient temperatures. The results show that pipe settlement of the supported pipe was 45% of settlement of the unsupported pipe. Settlement for the unsupported section was approximately 35% of the thaw bulb depth below the initial pipe elevation, only 30% of that for the supported pipe due to the influence of the supports. The final thaw bulbs extended approximately 3.6 and 1.6 times of the pipe diameter below the unsupported and supported pipe bottom elevations, respectively. The sandbag supports kept frozen during the test period because of cooling effect of the thermosyphons. The maximum bending stress induced over the 20 m span length from bearing of the full cover over the pipe would be equivalent to40% specified minimum yield strength(SMYS). Potential buckling of the pipe should be considered as the ground thaws.This study also offers important data for calibration and validation of numerical simulation models.

Thermal Modeling of Thermosyphon Integrated Heat Sink for CPU Cooling

A thermal model has been developed to study the thermal behavior of Thermosyphon integrated Heat Sink during CPU cooling. An Indirect cooling module has been experimentally studied and analyzed under steady state condition for both natural and forced convection. The thermal model is employed to determine the actual heat transfer and the effectiveness of the present model and compared it with the conventional cooling method and found that there is an appreciable improvement in the present model.

Numerical analysis on the thermal regimes of thermosyphon embankment in snowy permafrost area

Snow covers the road embankments in winter in high latitude permafrost zones. The effect of snow cover on embankments was simulated based on field measurements of boundary conditions and initial ground temperature profile in Mohe,China. The effect of thermosyphons on the embankment warmed by snow cover was evaluated by numerical simulations as well. The results indicate that the difference of thermal regimes between non-thermosyphon and thermosyphon embankments reaches to 22 m in depth below the ground surface. It is much warmer in the non-thermosyphon embankment body in winter. Affected by the snow cover, heat flux gradually spreads into the deep ground of the subgrade over time. The permafrost table under the slope toe of a thermosyphon embankment is 1.2 m higher than that of a non-thermosyphon embankment in the 20 th year. In addition, the permafrost table at the slope toe of a thermosyphon embankment is 26 cm deeper over 20 years. These results indicate that thermosyphons can greatly weaken the warm effect of snow cover. However,thermosyphons cannot avoid the degradation of permafrost under the scenarios of snow cover. Therefore, composite measures need to be adopted to keep embankment stability in snowy permafrost zones.

Temperature distribution of fluids in a two-section two-phase closed thermosyphon wellbore

Compared with a conventional single section two-phase closed thermosyphon （TPCT） wellbore, a two-section TPCT wellbore has better heat transfer performance, which may improve the temperature distribution of fluid in wellbores, increase the temperature of fluid in wellheads and even more effectively reduce the failure rate of conventional TPCT wellbores. Heat transfer performance of two-section TPCT wellbores is affected by working medium, combination mode and oil flow rate. Different working media are introduced into the upper and lower TPCTs, which may achieve a better match between the working medium and the temperature field in the wellbores. Interdependence exists between the combination mode and the flow rate of the oil, which affects the heat transfer performance of a two-section TPCT wellbore. The experimental results show that a two-section TPCT wellbore, with equal upper and lower TPCTs respectively filled with Freon and methanol, has the best heat transfer performance when the oil flow rate is 200 L/h.

A Loop Thermosyphon Type Cooling System for High Heat Flux

With rapid development of the semiconductor technology, more efficient cooling systems for electronic devices are needed. In this situation, in the present study, a loop thermosyphon type cooling system, which is composed mainly of a heating block, an evaporator and an air-cooled condenser, is investigated experimentally in order to evaluate the cooling performance. At first, it is examined that the optimum volume filling rate of this cooling system is approximately 40%. Next, four kinds of working fluids, R1234ze(E), R1234ze(Z), R134a and ethanol, are tested using a blasted heat transfer surface of the evaporator. In cases of R1234ze(E), R1234ze(Z), R134a and ethanol, the effective heat flux, at which the heating block surface temperature reaches 70°C, is 116 W/cm2, 106 W/cm2, 104 W/cm2 and 60 W/cm2, respectively. This result indicates that R1234ze(E) is the most suitable for the present cooling system. The minimum boiling thermal resistance of R1234ze(E) is 0.05 (cm2·K)/W around the effective heat flux of 100 W/cm2. Finally, four kinds of heat transfer surfaces of the evaporator, smooth, blasted, copper-plated and finned surfaces, are tested using R1234ze(E) as working fluid. The boiling thermal resistance of the blasted surface is the smallest among tested heat transfer surfaces up to 116 W/cm2 in effective heat flux. However, it increases drastically due to the appearance of dry-patch if the effective heat flux exceeds 116 W/cm2. On the other hand, in cases of copper-plated and finned surfaces, the dry-patch does not appear up to 150 W/cm2 in effective heat flux, and the boiling thermal resistances of those surfaces keep 0.1 (cm2·K)/W.

Effect of Non-Condensable Gas Leakage on Long Term Cooling Performance of Loop Thermosyphon

We have developed a loop thermosyphon for cooling electronic devices. The cooling performance of a thermosyphon deteriorates with an increasing amount of non-condensable gas (NCG). Design of a thermosyphon must consider NCG to provide guaranteed performance for a long time. In this study, the heat transfer performance of a thermosyphon was measured while changing the amount of NCG. The resultant performances were expressed as approximations. These approximations enabled us to predict the total thermal resistance of the thermosyphon by the amount of NCG and input heating. Then, using the known leakage in the thermosyphon and the amount of dissolved NCG in the water, we can predict the amount of NCG and the total thermal resistance of the thermosyphon after ten years. Although there is a slight leakage in the thermosyphon, we are able to design a thermosyphon with a guaranteed level of cooling performance for a long time using the proposed design method.

Performance Analysis of Solar Water Heater

Two solar water heaters of 100 liter and 200 liter were installed on the roof-top of electronics laboratory at Old Academic Building (OAB) of Electrical and Electronics Engineering Department of BUET. The data of 12 months have been collected and analyzed in this paper. It is found that the incoming hot tap water is about 30℃ higher than the room temperature during day time during winter months. This is about 25℃ in afternoon hours.

Numerical Simulation of Two-Phase Closed Thermosyphon

Heat transfer and hydrodynamics of two phase closed thermosyphon （TPCT） is studied using finite difference method of Stream function and vorticity. The mathematical model is formed for both vapor phase and liquid film in a non-dimensional form. The momentum and energy equations as parabolic equations have been solved by means of the locally one-dimensional scheme of A.A. Samarskii. The Poisson equation for the stream function has been approximated on the basis of the discrete scheme ＂cross＂. The obtained system of algebraic equations has been solved by the successive over relaxation method. The effect of the Rayleigh number on the velocity and temperature fields in the domain of interest and on the average Nusselt number at the solid-fluid interfaces is investigated. It was found that increase in the Rayleigh number leads to the increment of both the average Nusselt number and the stream function in the vapor zone.

Compact Thermosyphon Heat Exchanger for Power Electronics Cooling

The heat losses density in power electronics products follows an ever increasing trend. Nowadays they reach 200 W/cmz at chip level and 50 W/cm2 at heatsink base level. Water cooling is the most effective cooling method but unfortunately water is often undesired due to high voltages or costumer requirements. Two-phase cooling is a promising technology for electronics cooling. It allows using dielectric fluids in passive systems and still benefits from very high heat transfer coefficients. Thermosyphons are a particularly interesting technology in the field of power electronics because it is entirely passive and a simple equipment. ABB has developed a compact thermosyphon heat exchanger based on automotive technology, which uses numerous multi-port extruded tubes with capillary sized channels disposed in parallel and brazed to a heated base plate in order to achieve the desired compactness. The experimental performances of this novel power electronics cooling system are presented with R134a as a working fluid. The influence of several parameters on the performances was studied experimentally： coolant flow rate, coolant temperature, heat load and fluid filling.

Climate warming is likely to weaken the performance of two-phase closed thermosyphon on the Qinghai-Tibet Plateau

Over the years,numerous geotechnical approaches have been implemented to mitigate the adverse effects of climate warming on various infrastructures in the permafrost region of the Qinghai-Tibet Plateau(QTP),such as the Qinghai-Tibet Highway and Railway,and achieved the expected engineering outcomes.However,little attention has been given to whether the performance of these geotechnical approaches has changed during the ongoing process of climate warming.To investigate the performance variation of one of these geotechnical approaches,which is two-phase closed thermosyphon(TPCT),during sustained climate warming,we conducted a statistical analysis of soil temperature monitoring data in 2003-2020 from eight regular embankments and six TPCT embankments in our permafrost monitoring network.The results indicate that TPCT undeniably has a cooling effect on the permafrost beneath embankments,even rapidly eliminated previously formed taliks beneath embankment.However,further analysis reveals that the performance of TPCT has been weakening during sustained climate warming,which has confirmed by the re-forming of the taliks beneath embankment where they had been previously eliminated.Based on the current understanding,we attributed the weakening of thermosyphon performance to a significant reduction in the air temperature freezing index caused by ongoing climate warming.Through this study,we aimed to draw attention to the evolving performance of geotechnical approaches in permafrost regions amid climate warming,prompting necessary engineering innovations to address this situation and ensure the sustainable development of the permafrost region on the QTP.

Application of a concrete thermal pile in cooling the warming permafrost under climate change

Permafrost degradation caused by climate warming is posing a serious threat to the stability of cast-in-place pile foundations in warm permafrost regions.Ambient cold energy can be effectively utilized by two-phase closed thermosyphons(TPCTs)to cool the permafrost.Therefore,we installed TPCTs in a cast-in-place pile foundation to create a unique structure called a thermal pile,which effectively utilizes the TPCTs to regulate ground temperature.And we conducted a case study and numerical simulation to exhibit the cooling performance,and optimize the structure of the thermal pile.The purpose of this study is to promote the application of thermal piles in unstable permafrost regions.Based on the findings,the thermal pile operated for approximately 53%of the entire year and effectively reduced the deep ground temperature at a rate of at least-0.1℃per year.Additionally,it successfully raised the permafrost table that is 0.35 m shallower than the natural ground level.These characteristics prove highly beneficial in mitigating the adverse effects of permafrost degradation and enhancing infrastructure safety.Expanding the length of the condenser section and the diameter of the TPCT in a suitable manner can effectively enhance the cooling capability of the thermal pile and ensure the long-term mechanical stability of the pile foundation even under climate warming.

Thermal and mechanical characteristics of a thermal pile in permafrost regions

Cast-in-place pile foundations are widely used in permafrost regions to support buildings.The stability of cast-in-place pile foundations is highly sensitive to permafrost thermal regime changes.Permafrost degradation caused by climate change is increasing the disaster risk of castin-place pile foundations.However,proactive cooling methods for cast-in-place pile foundations are seldom reported.The cold energy produced by two-phase closed thermosyphons(TPCTs)can efficiently prevent the permafrost thermal regime from being disturbed by engineering activities and climate change.TPCTs were installed in a concrete pile forming a thermal pile.Then,a model experiment was conducted to explore the thermal regime,influence scope,dissipation process of cold energy,and freezing strength of the thermal pile.The results indicated that the thermal pile may significantly cool the foundation soil.Most of cold energy produced by the thermal pile dissipated during the warm period,and the cooling scope of the thermal pile can cover the area within a 40 cm(twice the pile diameter)radius around the pile.Additionally,the TPCTs can significantly improve freezing strength between the thermal pile and frozen soil.The lesson learned from this study can provide a new approach to control the thermal regime of cast-in-place pile foundation in permafrost,which was of valuable to the construction of pile foundations in cold regions.

Strengthening effect of crushed rock revetment and thermosyphons in a traditional embankment in permafrost regions under warming climate

The embankment of the Qinghai-Tibet Railway(QTR)has been experiencing severe problems due to permafrost degradation,especially in warm and ice-rich permafrost regions.Based on ground temperature and deformation data of the embankment at location K1497+150 of the QTR in 2006-2018,the thermal regime and deformation process were analysed.The results showed that 1)the degradation rate of permafrost under the embankment was faster than that under the natural site without engineering construction,and 2)the deformation rate of the embankment had exceeded the safety range for the QTR embankment before adopting mitigative measures.In addition,this study evaluated the cooling effect on the underlying permafrost and working mechanism of two strengthening measures.Two measures,crushed rock revetment(CRR)and thermosyphons,were adopted to protect the underlying permafrost from continual degradation.The individual cooling effect of the CRR was not as good as that of the thermosyphons.However,the combination of the two could enhance the protective effects of each on the underlying permafrost,effectively cooling the permafrost and improving the stability of the embankment.